The objective is to elucidate the nature of the strong association between the Epstein-Barr virus (EBV) and poorly differentiated nasopharyngeal carcinoma (NPC). Two specific questions are asked. 1. Does NPC-specific EBV exist? 2. Is the EBV indispensible in the pathogenesis of NPC? Answers to the two questions will be obtained by the following procedures. 1. EBV will be isolated from NPC tissues obtained in Hong Kong and China. EBV-derived from organ NPC biopsy will be compared with those derived from other NPC biopsies (i.e. other NPC patients). All the NPC-derived EBV will also be compared with those from healthy controls in Hong Kong and China and with those from infectious mononucleosis patients and healthy controls in Davis, California. The comparison will be based on DNA and antigenic analysis. 2. At least 100 NPC biopsies will be tested for cells positive for the EBV-associated nuclear antigens (EBNA). Specimens negative for EBNA will be tested for EBV DNA sequences. 3. All NPC patients who are negative for IgA anti VCA (EBV viral capsid antigen) will be tested for IgG anti VCA. Through the comparison of EBV isolates from diverse sources, data regarding similarities and differences among NPC-derived EBV and between NPC-derived EBV and EBV from other sources should become available. With these data, it should be possible to determine if NPC-specific EBV exists. After studying 100 or more NPC biopsies, one will have data on the presence of EBV in each biopsy. If one or more biopsies is free of EBV, the conclusion will have to be that EBV is not indispensable in the pathogenesis of NPC. If one or more NPC patient(s) is negative for IgG anti VCA, it is reasonable to conclude that the EBV is not an essential factor in the pathogenesis of NPC. (IgG anti VCA is a reliable indicator of past EBV infection; a negative IgG anti VCA reaction indicates no prior exposure to the EBV.)